In order to measure a flow rate of a fluid to be measured flowing through a flow tube, a vortex flowmeter and a thermal flowmeter are used.
As is generally known, the vortex flowmeter makes use of the fact that the number of Karman vortexes (vortex frequency) generated by a vortex generator in a unit time is proportional to a flow rate within a predetermined Reynolds number range regardless of whether the vortex generator is a gas or a liquid when the vortex generator is placed in a fluid flow. The constant of proportionality is referred to as the Strouhal number. As vortex detectors, a thermal sensor, a strain sensor, an optical sensor, a pressure sensor, an ultrasonic sensor and the like are given. The above vortex detectors can detect a thermal change, a change in lift or the like caused by the vortexes. The vortex flowmeter is a simple flowmeter capable of measuring a flow rate without being affected by physical properties of the fluid to be measured, and is widely used for flow rate measurement of gases or fluids (for example, see Japanese Patent No. 2869054).
The thermal flowmeter includes a temperature sensor (fluid temperature detecting sensor) and a heating temperature sensor (heating-side temperature sensor). A temperature of the heating temperature sensor (flow velocity sensor (heater)) having a function of a temperature sensor and a function of a heating sensor is controlled to have a constant difference in temperature with respect to a temperature measured by the temperature sensor. This is because a heat quantity, which is removed from a heater when the fluid to be measured is caused to flow, is correlated with a mass flow rate. The mass flow rate is calculated from electric power for heating the heater (for example, see Japanese Patent Application Laid-open No. 2004-12220).
Japanese Patent Application Laid-open No. 2006-29966 discloses a technology of a multi-vortex flowmeter including both the function of the vortex flowmeter and the function of the thermal flowmeter. The multi-vortex flowmeter is capable of measuring a flow rate ranging from an extremely low flow rate to a high flow rate with good accuracy, and is particularly superior to the other flowmeters in this point.
The multi-vortex flowmeter can selectively use the function of the vortex flowmeter and that of the thermal flowmeter according to the condition of a flow of the fluid to be measured flowing through a flow channel of a flow tube. Specifically, the function of the thermal flowmeter is used to perform a measurement in an extremely low flow rate region and a low flow rate region, whereas the function of the vortex flowmeter is used to perform a measurement in a high flow rate region.
Since a sensitivity of the vortex detector is insufficient in the vortex flowmeter when the flow rate is lowered to reduce a vortex differential pressure, control is performed to switch the function to that of the thermal flowmeter at a predetermined lower limit flow rate in the multi-vortex flowmeter.
In the conventional multi-vortex flowmeters, the control is performed to determine switching between the function of the vortex flowmeter and that of the thermal flowmeter based on the flow rate. Specifically, in the conventional multi-vortex flowmeters, the control is performed to effect the switching at a given flow rate. The inventor of the present application believes a problem lies in that a pressure in the flow tube is not taken into consideration at all. Hereinafter, the problem is described referring to the drawings.
The inventor of the present application has found that the lower limit flow rate serving as a criterion of determination for switching between the functions of the flowmeters can be lowered based on the fact that the vortex differential pressure increases when the pressure in the flow tube increases even at the low flow rate. The inventor of the present application intends to reflect the thus found result in the multi-vortex flowmeter. In order to make use of the advantage of the vortex flowmeter, the inventor of the present application intends to measure the flow rate by using the function of the vortex flowmeter as much as possible.
In FIG. 7(a), on a graph showing a volume flow rate [L/min] on an axis of ordinate and a pressure in the flow tube [Mpaabs] on an axis of abscissa, the minimum flow rate of the vortex flowmeter (alternate long and short dash line) is represented by a curve. This graph shows that the vortex flowmeter can measure a lower flow rate as the pressure in the flow tube increases. This is because the vortex differential pressure increases as the pressure in the flow tube increases even at the low flow rate, resulting in a stabilized vortex signal. This fact has been found by the inventor of the present application.
Meanwhile, the switching is performed at a given flow rate in the conventional multi-vortex flowmeters as described above. Therefore, when switching points are plotted on the graph, the switching point is obtained as represented by a horizontal thick solid straight line in FIG. 7(b) (it is natural that the switching points are plotted above the curve of the minimum flow rate (alternate long and short dash line) of the vortex flowmeter).
Next, the thermal flowmeter which satisfies the switching point (thick solid line) and the minimum flow rate (alternate long and short dash line) of the vortex flowmeter as illustrated in FIG. 7(b) is considered. For the selection of the thermal flowmeter, the thermal flowmeter having the maximum flow rate as represented by a curve (short broken line) in FIG. 6(a) which is plotted above the curve of the minimum flow rate (alternate long and short dash line) of the vortex flowmeter and does not cross the switching point (thick solid line) should be selected.
However, although the thermal flowmeter having the maximum flow rate as represented by the curve (short broken line) in FIG. 8(a) has a large flow rate measurement range, the thermal flowmeter retains the possibility of failing to provide sufficient accuracy in the extremely low flow rate region or the low flow rate region, which is necessary as the multi-vortex flowmeter.
At present, there are a large number of thermal flowmeters with good accuracy. The inventor of the present application believes that an improved multi-vortex flowmeter can be provided by selecting the thermal flowmeter having the curve of the maximum flow rate closer to the curve of the minimum flow rate (alternate long and short dash line) of the vortex flowmeter from the large number of thermal flowmeters. When the thermal flowmeter with good accuracy is selected, however, a curve of the maximum flow rate (long broken line) crosses the switching point (thick solid line) in the middle as shown in FIG. 8(b). As a result, a measurement impossible state is generated.
The inventor of the present application considers as follows. Specifically, the switching point which is not in consideration of the pressure in the flow tube is a factor which prevents the use of the thermal flowmeter with good accuracy. The inventor of the present application intends to use the thermal flowmeter with good accuracy to provide an improved multi-vortex flowmeter.
Moreover, the inventor of the present application has found the fact that the vortex differential pressure increases when the pressure in the flow tube increases even at the low flow rate and therefore the lower flow rate serving as the criterion of determination for switching the functions of the flowmeters can be lowered, and intends to reflect the thus found result in the multi-vortex flowmeter. The inventor of the present application intends to use the function of the vortex flowmeter to measure the flow rate as much as possible in order to make use of the advantage of the vortex flowmeter.